A major number of electron guns of the Pierce type in use today are gridded to allow beam current control and on/off switching by grid voltage levels roughly two orders of magnitude less than the applied cathode-to-anode voltage. Such guns are used in various electron beam devices, such as microwave and millimeter-wave tubes, linear accelerators, and ubitrons or free electron lasers.
A paper describing gridded electron guns has been published by co-inventor Richard B. True, "A Theory For Coupling Gridded Gun Design With PPM Focussing," IEEE Transactions On Electron Devices, Vol. ED-32, NO. 3, March 1984, pp. 353. This paper presents theoretical and experimental methods for the design and optimization of a gridded Pierce electron gun.
In addition to this paper, the co-inventor of the present invention, R. B. True, has described an Improved Dual-Mode Electron Gun in U.S. Pat. No. 4,593,230, issued June 3, 1986, filed Mar. 29, 1982, assigned to the same assignee as this invention. This application discloses an improved shadow grid and control grid configuration which permits an electron gun to operate in a dual mode.
The grid configurations shown in the True article and the True U.S. Pat. No. 4,593,230 disclose a grid formed by a plurality of apertures placed within a thin conductive sheet, for example, wherein the material left between the apertures forms grid vanes having a uniform width. When these grids are used in an electron gun to terminate the flow of electrons from the cathode toward the anode, there is a requirement for a relatively dense number of grid vanes, or a relatively high cut-off voltage, to sufficiently terminate the electron flow.